Air–sea CO2 exchange in the Baltic Sea—A sensitivity analysis of the gas transfer velocity

Abstract

Air–sea gas fluxes are commonly estimated using wind-based parametrizations of the gas transfer velocity. However, neglecting gas exchange forcing mechanisms – other than wind speed – may lead to large uncertainties in the flux estimates and the carbon budgets, in particular, in heterogeneous environments such as marginal seas and coastal areas. In this study we investigated the impact of including relevant processes to the air–sea CO 2 flux parametrization for the Baltic Sea. We used six parametrizations of the gas transfer velocity to evaluate the effect of precipitation, water-side convection, and surfactants on the net CO 2 flux at regional and sub-regional scale. The differences both in the mean CO 2 fluxes and the integrated net fluxes were small between the different cases. However, the implications on the seasonal variability were shown to be significant. The inter-annual and spatial variability were also found to be associated with the forcing mechanisms evaluated in the study. In addition to wind, water-side convection was the most relevant parameter controlling the air–sea gas exchange at seasonal and inter-annual scales. The effect of precipitation and surfactants seemed negligible in terms of the inter-annual variability. The effect of water-side convection and surfactants resulted in a reduction of the downward fluxes, while precipitation was the only parameter that resulted in an enhancement of the net uptake in the Baltic Sea. • Variability of air–sea CO 2 fluxes is modulated by mechanisms other than wind speed. • Gas transfer velocity parameterizations are not adequate for coastal/marginal seas. • Water-side convection enhances air–sea fluxes during winter in the Baltic Sea. • Surfactants suppress CO 2 downward fluxes during summer. • Precipitation represents a net increase of the carbon uptake.